This kind of bi-leaflet prosthetic heart valve is essentially known from German Offenlegungsschrift 31 28 704 or from International Patent Application WO 89/02254. Known bi-leaflet prostheses comprise a symmetrical structure and design in so far as the outline of both leaflets defines the same area or magnitude of surface area. The leaflets are arranged symmetrically with respect to a standard center plane on the valve ring plane. A longitudinal central axis of the single axle on which the two leaflets pivot is in alignment with one diameter of the valve ring.
The development of mechanical prosthetic heart valves started with the so called ball-cage-prostheses ("Starr-Edwards heart valve"). Development proceeded from the tilting disc prostheses ("Bjork-Shiley heart valve", "Medtronic-Hall heart valve", and "Omnicarbon heart valve") to the bi-leaflet prostheses ("St. Jude Medical heart valve", "Edward-Duromedics heart valve", "Carbomedics heart valve" and "Sorin-Bicarbon heart valve") mainly in use today.
With known bi-leaflet prosthesis heart valves, each leaflet comprises its own device for attachment and/or guidance. In the open position, both leaflets are arranged essentially parallel to and at a distance from each other. This type of arrangement provides essentially three flow zones. One flow zone exists between the leaflets and one flow zone exists between the valve ring and the face of the leaflet facing the blood flow. This arrangement allows a reduction of the pressure gradient and an improved response of the leaflet to any change in blood pressure and direction of blood flow. Also, the arrangement is accompanied by a corresponding reduction in regurgitation and closer resemblance of the blood dynamics to the physiology of the natural valve system.
However, there is still a problem of the risk of thrombo-embolism. This risk requires a continuously controlled administration of anti-coagulants. Further, the risk may include a persistent hemolysis that may frequently increase years after the valve replacement. In this connection, please refer also to D. Horstkotte, C. Aul and L. Seipel, "Einflu.beta. von Klappentyp und Klappenfunktion auf die chronische intravasale Haemolyse nach alloprothetischem Mitral--und Aortenklappenersatz" (Influence of valve type and valve function upon chronic intravasal hemolysis after alloprosthetic mitral and aortal valve replacement) in Z. Kardiol., 72, (1983), 119.
Hemolysis results from a damage of blood components being subject to a deformation (erythrocytes) and may be evidenced by the rise in LDH and/or by a drop in haptoglobin in the plasma. Among significant causes of hemolysis are high flow velocities (jet flow), areas of turbulence and turbulent flows, which cause high shearing stresses. The latter occur, in particular, in the boundary layers between adjacent flow zones comprising different velocity profiles. In addition, an asynchronous opening and closing behavior of the two leaflets of an artificial valve causes turbulence and turbulent flows. Moreover, as the leaflet comes closer to the valve ring during the action of closing, mechanical damage to the erythrocytes may be caused. In particular, in the aorta position, flow zones adjacent to the periphery and comprising a high flow velocity may impair the endothelium at the arch in the aorta. In addition, in the case of a symmetrical bi-leaflet aorta prosthesis, one must expect considerable regurgitation due to the duration of time required to close, and further due to the asynchronous closing movement of the two leaflets. In the mitral position, it is not possible to achieve a physiological flow profile in the ventricle with a bi-leaflet prosthesis of symmetrical form, because the two marginal flows impede each other. There is an in adequate scouring of the ventricle. The surgeon is unable to remedy this defect by rotating the symmetrical artificial valve in the plane of the valve ring, because he has to expect an interference with components of aorta valve apparatus.
Testing methods have been developed that allow dissolved and briefly excited molecules of photochromic dye stuffs to demonstrate the flow conditions in a pulsating liquid downstream of an artificial valve. See, for example, Yurechko, V. N. et al. in ASAIO Transactions, 1989, 35, 218-221, or in the International Journal of Artificial Organs, No. 6, 1993, 29-33. As an example, these investigations confirm on the distal side (downstream) of an open "Jude Medical heart valve" a flow profile comprising three distinctive flow zones, each having a different velocity profile.
Using the aforementioned test method of Yurechko, V. N., it was possible to show within the present invention, that relatively simple flow conditions may be obtained downstream of a bi-leaflet prosthesis, both leaflets thereof pivot on a common axis. In the open position the two leaflets are arranged in a V-shaped arrangement. Downstream of this arrangement there is a zone of stagnation, accompanied on each side by a flow zone. The extension of the stagnation zone may be influenced by the angle of opening of the leaflets and by the configuration of the leaflet side facing the flow. A relatively small stagnation zone may be achieved by matching these parameters. Consequently, this type of prosthesis allows a simpler flow profile than other types of bi-leaflet prostheses. A flow profile with only two flow zones creates less turbulence and, consequently, reduces the shearing stress upon the erythrocytes. The necessarily present opening angle of the two V-shaped arranged leaflets accelerates the closing action. A more rapid closing action reduces the loss of energy.
In addition, this form of construction provides for an attachment of the two leaflets to a common axle in a simple, stable and reliable manner. The axle or shaft may comprise an adequate diameter and may be secured reliably to the valve ring. The axle passes through a bore defined by a sleeve that is formed integrally in (a one piece manner) with the leaflet. This results in high stability and reliability. An unintentional removal or detachment of the leaflet is not possible. This is in contrast to previously reported accidents, for example, with the "St. Jude Medical heart valves" (see, for example, Journal of Thorac. Cardiovasc. Surgery, No. 31 (1983) and No. 86 (1983); or, in the case of "Edward-Duromedics heart valves", see, for example, Journal of Thorac. Cardiovasc. Surgery, No. 97 (1984), pages 90-94).
Due to the anatomical features, only a very few options are available when inserting an artificial valve. In most cases, the suture ring is attached to the natural valve ring or to remnants of the valves removed. The position of the coronary ostia and of the aortal seal provides further limitations with respect to the insertion of an aortal prosthesis. In the mitral position, the jet flow during the closing step of the aortal valve following the closing action of a mitral prosthesis has to be considered.
Due to the eccentric suspension of the tilting disc of a tilting disc prothesis, the tilting disc prosthesis provides an asymmetrical distal flow profile. The surgeon may use this fact when orientating the valve in order to come closer to the physiological conditions. See for example Viking O. Bjork and Dan Lindblom, "The Monostrut Bjork-Shiley Heart Valve", in Journal of the American College of Cardiology, Vol. 6, No. 5, 1142-1148 (November 1985). This essay indicates a preferred orientation of the tilting disc both in the aorta position and in the mitral position, providing corresponding effects on the volumes of flow in the sections of the flow channel cross-sections in the plane of the valve ring and upon the distal flow profile. This possibility is not provided in the case of bi-leaflet prosthetic valves of symmetrical structure or design.
As stated above, a bi-leaflet prosthesis, particularly a bi-leaflet prosthesis wherein both leaflets pivot on a common axis, provides certain advantages compared with a tilting disc prosthesis. However, in addition to the selection of size (valve ring diameter) it would be desirable to provide the surgeon with a further possibility to affect the flow conditions with such a bi-leaflet artificial valve.